The invention resides in a method for processing ashes of waste incineration plants and of mineral residues by desalting and artificial aging by means of electrodynamic underwater processes and to an apparatus for performing the method.
Ashes of waste incineration plants are at first considered to be a toxic material which must be safely disposed. In order to reduce costs, ways are being examined of utilizing the ashes produced by waste incineration plants in some economic way so that they do not need to be deposited at relatively high costs. In this regard, the German government has issued regulation document TA concerning municipal waste (TASI). According to this regulation document, the ashes must be stored first for some time (3 months) in deposits made particularly for that purpose in a way that the deposited ashes are isolated from ground water. The expenses for the treatment of the ashes are at this point about DM 40.xe2x80x94per ton of ashes.
The classification of ashes as toxic material is the result of three factors:
1. The increased salt content
2. The possibility of elutriation of heavy metals or anions
3. The incomplete incineration resulting in a high content of carbon (TOC-Total Organic Carbon).
The following table shows the admissible values, separated by classes in comparison with the values of fresh ashes from waste incineration plants.
The high content of soluble lead in the fresh ash is particularly noticeable. The reason herefor is that lead salts like other heavy metal salts are easily dissolvable under highly basic conditions, that is, at high pH values. The high lead content is what requires the interim storage. During storage, the ash becomes chemically more stable by exposure to oxygen and carbon dioxide. The binding of basic calcium hydroxide and the conversion into carbonate by carbon dioxide are particularly pointed out as these chemical reactions reduce the pH value. In addition, lead is stabilized during the interim storage period by forming water insoluble lead carbonate.
The waste incineration plants in Germany presently produce 3xc3x97106 kg ashes per day. In comparison with other countries, a substantial increase is expected for the future.
After having been stored, the ashes are used mainly as road bed material in the construction of new roads, where the ashes do not come into contact with ground water.
It is the principal object of the present invention to provide a method for rapidly converting the ashes such that an interim storage is no longer necessary, but that the ashes can be used at least in deposit class 1.
In a method and apparatus for processing materials comprising ashes from waste incineration plants and mineral residues to condition them in a short time by washing and by electro-dynamic processes and by shock waves generated by high voltage spark discharges, the material is made chemically inert and aged so that it can be used immediately after treatment without harming the environment.
In the process, the ashes are artificially aged. For this purpose, the ashes are supplied to a liquid-filled reaction container. The liquid in the reaction container is generally water, but other liquids suitable for the process may be used. While the ashes are transported upwardly to the reaction container, the ashes are subjected in a countercurrent flow arrangement to process liquid so that the salts disposed on the surface of the ash particles are flushed off and dissolved. The salt-enriched liquid is collected in a storage tank and the saturated liquid is finally supplied to an evaporator for the precipitation of the salts.
In the reaction container, two insulated spaced electrodes are disposed or a single electrode is arranged opposite a counter electrode which is disposed in the reaction container and grounded therein. The opposite high voltage electrode is uninsulated at its distal end over a length (about 3 cm) as suitable for the process. The blank metallic ends or tips of the two electrodes or of a tip disposed opposite a flat electrode surface area are spaced from one another as suitable for the process by a distance which is 2-7 cm. Experience shows that good results can be achieved if the grounded electrode is disposed below the high voltage electrode and is in the form of a sieve. However, also, other electrode arrangements are possible. The submerged electrodes may be arranged in parallel or they may be disposed at an acute angle with respect to each other. However, the most suitable arrangement is selected depending on the particular process and may be optimized in a known manner by test discharges.
1. In order to prevent fragmentation of the ashes a sieve arrangement with a wide mesh width of about 40 mm and an opening of about 100 mm diameter are used. With an adjustable repetition rate of the high voltage spark discharge, each particle may be exposed to a number of shocks during downward movement through the sieve and the opening. The size of the ash particles should generally not be reduced. Therefore the electrodes or at least their end areas are surrounded by dielectric nets, which keep the particles at a distance.
2. For an optimal process performance, a set of operating parameters is determined before the actual processing. This concerns the discharge voltage, the impulse duration, the energy of the industrial discharges and the repetition rate of the discharges.
3. The following parameters have been found suitable:
an electric field strength of 100 to 300 kV/cm,
a pulse increase time of not more than 500 xcexcg and
spark generating a shock wave by a release of 120-400 J per cm discharge path (see also DE 195 43 232).
With this set of parameters an economical operation with minimal specific energy consumption and, at the same time, minimal wear of switches in the high voltage pulse generator and the electrode system is achieved.
The shock wave generated during the electrical discharge between the two electrode tips causes the almost complete transfer to, and dissolution in, the process liquid of all the compounds which are soluble in the process liquid. This solution is then conducted away for further processing.
The liquid treatment of the ashes is performed in counter current flow to the continuous movement of the ashes through the treatment apparatus. The shock-treated ashes are washed during transport from the reaction container to a holding area or to a containment including fresh process liquid and, in the process, are again washed in a counter current procedure in order to wash out the remaining toxic materials. The process liquid enriched with the residual toxic material is conducted to the reaction container where it receives additional liquid-soluble substances. From an overflow at the reaction container, the process liquid is removed and conducted to the end of another transport device by way of which the ashes to be processed are carried to the reaction container. From the end of this transport device up to its beginning the process liquid flows over the surface of the particles to wash the particles. Finally, at the beginning of the transport device, the process liquid flows as a saturated liquid into a collection containment from where it is conducted away for the precipitation of the substances dissolved therein.
With each discharge and the shock wave generated thereby, hydroxyl radicals are formed in the process liquid which provide for oxidation and passivation of the ash particles contained in the liquid. Carbon dioxide is also introduced into the process liquid. This causes precipitation of calcium hydroxide, whereby the pH value of the process liquid is lowered and, as a result, the solubility of the heavy metals is reduced. It also results in a reduction of the time required for processing the ashes.
Samples of fully treated material are removed from the eluate and the residual heavy metal and the toxic material concentration therein is determined. This shows the efficiency of the method and the apparatus used therewith: The concentration reduction of heavy metal and toxic materials is such that the ashes will pass immediately after processing at least for deposition class 1 utilization.
An apparatus for performing the method according to the invention will be described below with reference to the accompanying drawings.